1. Field of the Invention
The present invention relates to a rolling bearing and more particularly to a rolling bearing which is improved in both fatigue fracture strength and rolling contact fatigue life. The rolling bearings are designed for supporting, for example, rocker arms for opening and closing the intake valve or exhaust valve in an automobile engine or supporting planet gears in a planetary gear mechanism.
2. Description of the Background Art
Such rolling bearings for supporting the rocker arms and the planet gears in a planetary gear mechanism are now in demand for sustaining, as a full complement roller bearing with the use of no cage, a heavier, high-speed load. This specification does not distinguish the full complement roller bearing from any other rolling bearing but refers to the rolling bearing comprehensively. In the rolling bearing with no cage, rolling elements may collide against each other and thus an insufficient lubrication can cause surface origin flaking of rolling elements or raceway surfaces.
Also, when the rollers run at higher speeds, their surfaces can be damaged due to mounting error or unbalanced load or they can skew in the movement with no proper positional controlling. This will result in surface origin flaking due to slippage or subsurface origin flaking due to a local increase in operating surface stress. As a result, the heat by slippage and the local increase in operating surface stress can cause surface damage such as peeling, smearing, surface origin flaking, and subsurface origin flaking depending on the load being applied.
Some schemes listed below have been proposed for eliminating the above drawbacks.
(1) A bearing for supporting a cam follower in a valve driving mechanism of an engine is disclosed in Japanese unexamined patent publication 2000-38907 and has a calculated life increased up to 1000 hours or more at the rated number of engine revolutions.
(2) A bearing for supporting a cam follower in a valve driving mechanism of an engine is disclosed in Japanese unexamined patent publication H10-47334 and contains 10% to 25% of carbide, the dissolution of retained austenite decreased to 1/10 to 3/10 the original, HV830 to HV960 in the Vickers hardness at its ends and not longer than 25 μm in the average wavelength for surface roughness, where its steel material is subjected to carbonitriding and hard-shot peening.
(3) A cam follower shaft disclosed in Japanese unexamined patent publication H10-103339 is coated with a solid lubricant layer of polymer compound for improving the anti-friction property.
(4) A cam follower shaft disclosed in Japanese unexamined patent publication H10-110720 is made of tool steels and subjected to ion nitriding and ion plating at a temperature lower than a tempering temperature for an increased hardness.
(5) In a bearing for supporting a cam follower shaft in a valve driving mechanism of an engine disclosed in Japanese unexamined patent publication 2000-38906, bending stresses imposed on the shaft is limited to not higher than 150 MPa.
(6) In a cam follower in a valve driving mechanism of an engine disclosed in Japanese unexamined patent publications 2000-205284 and 2002-31212, the raceway surface of a bearing component is protected with a phosphate coating which is improved in the lubricant retaining ability.
(7) In a cam follower in a valve driving mechanism of an engine disclosed in Japanese unexamined utility-model publication S63-185917, a shaft surface on which rollers roll is crowned.
(8) Japanese unexamined patent publication 2002-194438 discloses a shaft having a raceway surface subjected to high-density carburizing or carbonitriding for increasing the concentration of carbon to 1.2% to 1.7% while its core has the Vickers hardness number of HV300.
However, the cam follower provided with a roller in an engine is often attached to a rocker arm in such a manner that opposite ends of its shaft are deformed as by swaging and being anchored to the rocker arm. The shaft ends have to remain soft for ease of the swage locking while the central portion providing raceway surface is required to have an increased hardness. Some schemes have also been developed for the purpose (See Japanese unexamined patent publications H5-321616 and S62-7908 and Japanese examined patent publications H6-15811 and H6-80287).
Similarly, an inner member of a bearing in a planetary gear mechanism is also required to have an increased hardness at its raceway surface while at opposite ends it should remain soft for ease of the swage locking. Some schemes for the purpose have been developed (See Japanese Patent Laid-open Publications No. 2003-301933 and 2004-003627).